1. Field of the Invention
The present invention relates to an inkjet print head suitable for use in printers, facsimiles, plotters, bar-code printers, digital copying machines and the like and an inkjet printer having such an inkjet print head. Particularly, the present invention concerns an inkjet print head and inkjet printer which can respond to image signals to inject ink through thin nozzles, the injected ink being deposited on a recording medium such as paper and the like.
2. Description of the Related Art
Conventional inkjet printers are classified into the continuous type and the on-demand type. Ink used may be water based or oil based.
The continuous type inkjet printers continuously inject the ink through nozzles, the ink not used on recording being collected and re-used. The continuous type inkjet printers have higher head responsiveness, but are complicated and expensive, requiring an ink collecting mechanism. On the other hand, the on-demand type inkjet printers inject the ink on demand. The on-demand type inkjet printers have lower head responsiveness, but are more simple and inexpensive in structure.
The on-demand type inkjet printers are further divided into an electrostatic attraction force type in which the ink is drawn out through the nozzles under the action of electrostatic force and a pressure pulse type known as an in-pulse jet in which the ink is pressurized in and ejected from a pressure chamber through the nozzles.
The pressure pulse type inkjet printers are still further classified into piezoelectric type and bubble type. The piezoelectric type inkjet printers pressurize the ink by the use of a piezoelectric (electric strain) element and are further divided into single-chamber type in which the ink is supplied through a pressure chamber and two-chamber type which comprises a pressure chamber and an ink supply chamber.
The single-chamber type inkjet printers are further divided into Kyser type in which a planar pressure chamber is used and Zoltan type in which a cylindrical pressure chamber is used. The two-chamber type inkjet printers include Stemme type inkjet printers in which the ink is supplied directly near the nozzles.
FIG. 1 is a view showing the basic principle of a piezoelectric type head unit in the Kyser type inkjet printer. The head unit comprises a base plate 101 and ink supply passage 103, pressure chamber 102 and nozzle 106 which are formed in the surface of the base plate 101 so as to communicate with one another. The surface of the base plate 101 includes a vibrating plate 107 disposed thereon. The top of the vibrating plate 107 supports a piezoelectric element 108 which is disposed opposite to the pressure chamber 102. A signal generator 111 is connected to the piezoelectric element 108 such that a voltage is applied across the opposite sides of the piezoelectric element 108. The ink supply passage 103 is connected to an ink vessel 109 through a pipe 104.
In such an arrangement, when the voltage is applied from the signal generator 111 to the piezoelectric element 108, the piezoelectric element 108 causes the vibrating plate 107 to bend so that the ink 110 will be injected from the pressure chamber 102 through the nozzle 106 toward a recording medium 115 in the form of an ink droplet 110a.
After the ink has been injected and when the piezoelectric element 108 is released from the voltage, the vibrating plate 107 returns to its original position and a new amount of ink corresponding to the amount of the injected ink will be replenished from the ink vessel 109 through the ink supply passage and pipe 103, 104 under the capillary action of the nozzle.
FIG. 2 is a cross-sectional view of a conventional inkjet print head which utilizes the aforementioned piezoelectric type head unit. The piezoelectric type head unit is fixedly mounted on a base 112 through a holding frame 114. More particularly, the piezoelectric type head unit is first placed on the base 112. The holding frame 114 is cylindrical and includes an inwardly directed flange 114a formed therein at one end. The holding frame 114 is then positioned to engage a peripheral shoulder 112a in the base 112. The holding frame 114a includes threaded holes which are screwed by bolts 116 passed through through-apertures 114 in the base 112 to fix the holding frame 114 relative to the base 112. Thus, the outer periphery of the base plate 101 in the piezoelectric type head unit will be firmly held against the base 112 by being engaged by the inwardly directed flange 114a of the fixed holding frame 114. The piezoelectric type head unit is further urged and held against the holding frame 114 by a resilient member 117 which is received in a recess 112b formed in the base 112. A drive voltage is applied to the piezoelectric element 108 through flexible cables 109.
FIG. 3 is a cross-sectional view of another conventional inkjet print head which utilizes the piezoelectric type head unit. The base plate 101 of the piezoelectric type head unit is first placed in a counterbored hole 118a which is formed in a cylindrical base 118 at one end. The base plate 101 is then threadedly screwed to the base 118 through a machine screw 119. In such a manner, the piezoelectric type head unit is fixedly mounted on the base 118.
A piezoelectric type head unit including pressure chambers and piezoelectric elements arranged in a circle as shown in FIG. 4 will further be described below.
As shown in FIG. 4, the piezoelectric type head unit comprises a base plate of photosensitive glass, a plurality of radial pressure chambers 102 formed in the base plate, a common ink passage 103 formed in the base plate to surround and communicate with the pressure chambers 102, an ink inlet port 5 formed in the base plate for supplying the ink to the common ink passage 103 and a plurality of nozzles formed substantially centrally through the base plate. The pressure chambers, common ink passage and ink inlet port may be machined in the base plate by any suitable manner such as etching. The number of nozzles per unit area can be increased by radially forming the pressure chambers 102. A piezoelectric element 108 may be made of PZT or the like and mounted on the base plate at a position opposite to a vibrating plate corresponding to the respective one of the pressure chambers 102.
Each of the piezoelectric elements 108 is connected to a signal generator through a flexible cable means such that a voltage is applied to that piezoelectric element. The flexible cable means has an electrode means connected to the piezoelectric element and another electrode means for grounding the vibrating plate. As shown in FIGS. 4 and 5, a flexible cable 59 of the prior art includes a flexible cable tip portion 59a which has grounding electrodes 50' formed in corners of the tip portion 59a.
Since the inkjet print heads of the prior art as shown in FIGS. 2 and 3 are not very well supported near the center of the vibrating plate 107 opposite to the nozzles 106, the flexibility in the vibrating plate part corresponding to one or more energized piezoelectric elements causes unnecessary flexing in the vibrating plate part corresponding to the not-energized piezoelectric element adjacent to an energized piezoelectric element. This will provide an unstable discharge of ink droplets to reduce the quality of printing and to generate noise. The unnecessary resonance in the vibrating plate reduces the service life of the head and also provides various other problems.
In order to overcome the above problems, the piezoelectric element may be pressurized by such a resilient member 117 as is shown in FIG. 2. However, the vibrating plate is also pressurized through that piezoelectric element. When the pressure increases, therefore, the output of the ink will be decreased to reduce the energy effective in discharge.
In the flexible cable of the prior art, the grounding electrodes 50' are formed in the corners of the tip portion 59a, as described with reference to FIGS. 4 and 5. Thus, the area of the electrodes is not increased. As a result, a so-called "floating" in the grounding electrode may be produced to reduce the amount of electric current in the flexible cable and also the distance between the grounding electrode and the corresponding piezoelectric element electrode may be increased in some parts to increase the voltage drop and to so heat the head. The voltage cannot be applied by the floating in the grouding electrode and the resistance between the grounding electrode and the piezoelectric element electrode is varied due to the different distance between the grounding electrode and the corresponding piezoelectric element electrode to render the voltage drop variable. Thus, the piezoelectric element will be flexed with different deflections to render the change of volume in the pressure chamber variable. Accordingly, the diameter and speed of the discharged ink droplets become variable resulting in destabilizing the discharge characteristic of the ink droplets.